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<Title>Boost Graph Library: EventVisitor</Title>
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<H1>EventVisitor Concept</H1>

This concept defines the interface for single-event visitors. An
EventVisitor has an apply member function (<tt>operator()</tt>) which
is invoked within the graph algorithm at the event-point specified by
the <tt>event_filter</tt> typedef within the
EventVisitor. EventVisitor's can be combined into an <a
href="./EventVisitorList.html">EventVistorList</a>.

<p>
The following is the list of event tags that can be invoked in BGL
algorithms. Each tag corresponds to a member function of the visitor
for an algorithm. For example, the <a
href="./BFSVisitor.html">BFSVisitor</a> of <a
href="./breadth_first_search.html"><tt>breadth_first_search()</tt></a>
has a <tt>cycle_edge()</tt> member function. The corresponding tag is
<tt>on_cycle_edge</tt>. The first argument is the event visitor's
<tt>operator()</tt> must be either an edge or vertex descriptor
depending on the event tag.

<pre>
namespace boost {
  struct on_initialize_vertex { };
  struct on_start_vertex { };
  struct on_discover_vertex { };
  struct on_examine_edge { };
  struct on_tree_edge { };
  struct on_cycle_edge { };
  struct on_finish_vertex { };
  struct on_forward_or_cross_edge { };
  struct on_back_edge { };
  struct on_edge_relaxed { };
  struct on_edge_not_relaxed { };
  struct on_edge_minimized { };
  struct on_edge_not_minimized { };
} // namespace boost
</pre>


<h3>Refinement of</h3>

<a href="../../utility/CopyConstructible.html">Copy Constructible</a>
(copying a visitor should be a lightweight operation).

<h3>Notation</h3>

<Table>
<TR>
<TD><tt>G</tt></TD>
<TD>A type that is a model of <a href="./Graph.html">Graph</a>.</TD>
</TR>

<TR>
<TD><tt>g</tt></TD>
<TD>An object of type <tt>G</tt>.</TD>
</TR>

<TR>
<TD><tt>V</tt></TD>
<TD>A type that is a model of EventVisitor.</TD>
</TR>

<TR>
<TD><tt>vis</tt></TD>
<TD>An object of type <tt>V</tt>.</TD>
</TR>

<TR>
<TD><tt>x</tt></TD>
<TD>An object of type
<tt>boost::graph_traits&lt;G&gt;::vertex_descriptor</tt>
or <tt>boost::graph_traits&lt;G&gt;::edge_descriptor</tt>.</TD>
</TR>

</table>

<h3>Associated Types</h3>

<Table border>

<TR>
<TD>Event Filter </TD>
<TD><TT>V::event_filter</TT></TD>
<TD>
A tag struct to specify on which event the visitor should be invoked.
</TD>
</TR>

</table>

<h3>Valid Expressions</h3>

<Table border>

<tr>
<th>Name</th><th>Expression</th><th>Return Type</th><th>Description</th>
</tr>

<tr>
<td>Apply Visitor</td>
<td><TT>vis(x, g)</TT></TD>
<TD><TT>void</TT></TD>
<TD>
Invokes the visitor operation on object <tt>x</tt>, which is
either a vertex or edge descriptor of the graph.
</TD>
</TR>


</table>


<h3>Models</h3>

<ul>
 <li><a
 href="./predecessor_recorder.html"><tt>predecessor_recorder</tt></a>
 <li><a href="./distance_recorder.html"><tt>distance_recorder</tt></a>
 <li><a href="./time_stamper.html"><tt>time_stamper</tt></a>
 <li><a href="./property_writer.html"><tt>property_writer</tt></a>
 <li><a href="./null_visitor.html"><tt>null_visitor</tt></a>
</ul>

<h3>See Also</h3>

<a href="./EventVisitorList.html">EventVisitorList</a>,
<a href="./visitor_concepts.html">Visitor concepts</a>

<br>
<HR>
<TABLE>
<TR valign=top>
<TD nowrap>Copyright &copy; 2000-2001</TD><TD>
<A HREF="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</A>,
Indiana University (<A
HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br>
<A HREF="http://www.boost.org/people/liequan_lee.htm">Lie-Quan Lee</A>, Indiana University (<A HREF="mailto:llee@cs.indiana.edu">llee@cs.indiana.edu</A>)<br>
<A HREF="https://homes.cs.washington.edu/~al75">Andrew Lumsdaine</A>,
Indiana University (<A
HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
</TD></TR></TABLE>

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